Various types of manufactured articles are or can be made by the vacuum deposition of a material, for example, aluminum, onto a substrate in which the material is to be formed as a patterned layer on the substrate. A nameplate is one example of this type of article, which can have a plastic film substrate and a metallized layer on a surface thereof which is patterned to provide alphanumeric informational graphics or a decorative design. The nameplate may be applied to an object, such as by adhesive, for identification purposes; also, the same type of construction can be used as a faceplate for a switch or keyboard with the alphanumeric graphics used to denote specific switch sites. Another example of this general type of article is a membrane switch, including at least two layers of plastic film spaced from one another with conductive patterns on the two facing surfaces of the films. The conductive patterns will include one or more switch sites with a conductive lead extending from each site onto a tail panel for connection to external drive and sense circuits. The conductive patterns can be formed on the films by vacuum deposition techniques.
Vacuum deposition, which is well known, is the process of applying a material onto a substrate in a vacuum chamber by evacuating the chamber to the desired pressure by means of vacuum pumps and evaporating or sputtering a supply of the material within the chamber so as to coat the substrate. Metals that can be applied by vacuum deposition include aluminum, titanium, vanadium, chromium, nickel, copper, zirconium, niobium, molybdenum, palladium, silver, tin, tantalum, tungsten and gold. Compounds can also be applied by vacuum deposition, such as titanium nitride, tantalum nitride, silicon dioxide, aluminum oxide and Teflon (Reg. T.M.), by the sputtering technique. The vacuum in the chamber is normally in the range of 1.times.10.sup.4 torr when employing evaporation of the material, 1 to 4.times.10.sup.3 torr for magnetron sputtering and a lower pressure when using ion beam sputtering. The thickness of the layer of material formed on the substrate can vary widely, but vacuum deposition is most often used to apply thin layers up to about 0.0025 mm thick.
The vacuum deposition of a layer that completely covers a surface of the substrate is a relatively simple technique. However, when the layer is to be in the form of a pattern in which some parts of the surface of the substrate are to be covered and other parts are not to be covered, more difficult methods are required.
Several techniques are well known in the art for forming a patterned metallized layer on a substrate. The photoresist system is one such technique in which the entire surface of the substrate is metallized, a photoresist coating is applied over the meal layer, a photographic negative of the desired pattern is placed on top of the photoresist coating and exposed to light so as to polymerize parts of the coating, after which unexposed parts of the photoresist coating are washed away. The metal layer is removed from areas uncovered by washing away the unexposed photoresist coating by wet etching techniques using acidic or basic solutions or dry etching techniques using gas in the form of a plasma or excited by a photon beam (see Fonash, "Advances in Dry Etching", Solid State Technology, Jan. 1985, pp. 150-158). The exposed photoresist coating is then removed with solvents to provide a metallized pattern on the substrate. The photoresist technique can be practiced in either the "negative" form in which the photoresist coating is exposed in parts that are not to be metallized in the final pattern, as described, or in the "positive" form in which the photoresist coating is exposed in the parts that are to be metallized. Another technique is the mask system in which a mask having cut-out portions defining parts of the substrate to be metallized is applied to the substrate. Metal is then vacuum deposited onto the substrate only in the cutout portions of the mask, leaving the remainder of the substrate surface free of metal. All of these methods are well known in the art and are described, for example, in Handbook of Thin Film Technology, Maissel & Glang, published by McGrawHill Book Company, 1970.
The foregoing methods have a number of disadvantages. Photoresist methods are multi-step processes which are time-consuming; some use corrosive, hazardous materials that require safety precautions in their use and present disposal problems; substrate materials can be damaged during processing; and solvents used to remove materials can remove inks printed on the substrate. Mask systems require the use of costly masks, particularly with complicated patterns; it is difficult to obtain accurate registration between the mask and the substrate and also to obtain good contact between the mask and the substrate, which can cause shadowing; the dimensional expansion of metal masks when heated poses a problem; metal masks become heated during vacuum deposition and can melt plastic substrates; and it is difficult to make a mask that will provide a pattern with fine detail and resolution and get reliable transfer of the pattern to the substrate.
Another system for forming a patterned metallized layer on the substrate is described in U.S. Pat. No. 3,935,334. The process disclosed by this patent involves coating the substrate with a water-soluble paint that consists of a water-soluble polymer. Numerous polymers are said to be suitable, including polyvinyl alcohol, some cellulose polymers, polyvinyl pyrrolidone, polyacrylic ester polymers, etc. The process involves first coating the substrate layer with the water-soluble polymer in the desired pattern, vacuum depositing metal over the coated substrate, and then washing the substrate with water to remove the polymer coating and any metal deposited thereon. However, I have noted two problems with the process as described by this patent. First, coatings of water-soluble polymers described as being useful in the patent take such a long time to remove by the final water washing step as to make the process of little or no utility for production purposes, particularly production techniques which must be run at a relatively high speed in order to be economically feasible. The polymers have a slow dissolving rate and it is difficult to penetrate the metal layer over the polymer coating. Secondly, only a very thin layer of the water-soluble polymer can be applied to the substrate, which the patent states is preferably only about 0.003 mm thick or less. Apparently the thickness of the water-soluble layer is limited because the slow rate at which the polymer is dissolved limits the process speed.
The deficiencies of the prior art processes as described above provided the impetus for the research work that led to the development of the present invention. One of the main objects of this invention was to develop undercoatings that could be rapidly removed after the vacuum deposition of material over the coatings. A more specific object was to develop coatings based on water-soluble polymers but including other ingredients that would speed up the removal of the coatings when washing with water. Another specific object was to develop processes for vacuum metallization of substrates that involve the use of the new coatings described hereinafter.